A synchronous machine is a dynamoelectric machine, which may be utilized as a motor for driving a shaft or any load at a constant speed or as a generator for producing a voltage at a predetermined frequency depending on the speed of the driving shaft. When the device is used as a synchronous generator, it is customary to e.g. provide field excitation for the rotor through a synchronous brushless exciter generator. The exciter generator converts the direct current (DC) stator field into a polyphase alternating current (AC) armature voltage, which is rectified by a set of rotating rectifiers mounted on or within the driving shaft to provide the DC excitation for the field windings of the synchronous generator, i.e. for the rotor of the generator.
Thus, a rotating exciter is a reversed generator with the field winding, fed with DC current, implemented on the static parts. The armature is located on the rotating part and produces AC voltage. A set of parallel diodes with reversed polarity is used to rectify it to produce a DC current, required to energise the field winding of the synchronous machine, i.e. the rotor.
As an order of magnitude, the power generated by the exciter is 0.5 to 2% of the rated power of the synchronous machine. Because the rotating exciter is a generator, this power could as well be made available with low, medium or high voltage. The voltage/current balance is chosen to best fit the available diodes characteristics. In principle the limitation in output current, due to the available diodes, can be overcome by setting two or more diodes in parallel. Actually this would result in a strong overload on diodes, and therefore the need for a heavy down-rating of them.
Nowadays, usually rotors operate at speeds of 3000 rotations per minute, leading to currents in the range of 2000 Ampere for energising the field winding of the synchronous machine. Usually the synchronous machine cannot be operated at higher rotational speeds due to a number of limitations such as instability of the shaft as well as high centrifugal forces on the rotating parts. In power generation, at a specified output, an increase of the rotary speed of a turbine however is associated with a decrease in size and costs. Efficiency, too, can be improved. Therefore power generation turbines up to 70 MW are already connected to generators by way of gearing arrangements, so as to allow operation at higher rotary speeds. Consequently, there is a need for rotors for exciters that can be operated at high rotational speeds, which usually means lightweight construction, smaller diameter as well as longer dimension in axial direction and at the same time high stability with respect to centrifugal forces.